Abstract
We investigated localization of damage within the knee extensors (KEs) and plantar flexors (PFs) induced by downhill running (DR) by using transverse relaxation time (T2)-weighted magnetic resonance imaging (MRI). Fourteen young adults performed 45-min DR (−15% slope) at their maximal tolerable velocity. At pre- and 24, 48, and 72 h post-exercise, T2-MRI was scanned and T2 values for each muscle composing KEs and PFs at proximal, middle, and distal sites were calculated. Maximal isometric torque and rate of torque development (RTD: 0–30, 0–50, 0–100, 0–200 ms) were also measured. Maximal torque significantly decreased in KEs (14–17%) and PFs (6–8%) at 24–48 h post-exercise, with greater reductions for KEs. RTD in all phases, except for 0–200 ms in PFs, significantly decreased in KEs (11–42%) and PFs (13–23%) at least at one time point post-exercise. T2 significantly increased at several sites (3–5%) in both muscle groups at 24 and/or 48 h post-exercise. Among the T2-increased sites, the peak effect size (Cohen’s d) regarding T2 change was pronounced at proximal (1.05) and middle (1.64) vastus intermedius compared to the other sites (0.72–0.77). These results suggest that DR induces damage in both KEs and PFs, and especially affects proximal–middle sites of the vastus intermedius.
Highlights
Trail running and some marathon races involve level and uphill and downhill running (DR)[1]
The main findings of the present study were that 1) DR induced significant decreases in maximal torque in both knee extensors (KEs) and plantar flexors (PFs) at 24 and 48 h post-exercise, with greater reductions for KEs than PFs, 2) rate of torque development (RTD) in all phases significantly decreased at all time points post-exercise in KEs, while RTD50 at 24 h and RTD200 at all time points did not significantly decrease in PFs, and 3) T2 significantly increased at several sites of the KEs and PFs at 24 and/ or 48 h post-exercise, with pronounced effect size found at the proximal and middle vastus intermedius (VI) compared to the other sites
These results refuted our first hypothesis that a T2 increase would be pronounced at the middle site of the bi-articular muscles (RF and lateral gastrocnemius (LG)/medial gastrocnemius (MG)), while at least indirectly supporting the second hypothesis that DR would induce greater damage in the KEs than PFs
Summary
Trail running and some marathon races involve level and uphill and downhill running (DR)[1]. Several studies have reported that muscle damage is induced by DR but not by level or uphill running[10, 16], or very little[13], if any With such a background, DR has often been adopted as an exercise model to induce damage in leg muscles, with particular focus on the knee extensors (KEs)[10, 12, 18, 19]. Given that the knee joint performs greater negative (eccentric) work than the ankle joint does during DR2, we hypothesized that muscle damage would be greater for the KEs than the PFs
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